The Aeronautical Telecommunications Network (ATN) will shoulder no small amount of responsibility. Quite simply, it is being developed to interconnect all operational components of civil aviation, both air and ground, through a seamless worldwide network. Planned through the International Civil Aviation Organization (ICAO) process, ATN is forecast to be the critical enabling technology for Free Flight and all the other facets of the future era of communications, navigation, surveillance and air traffic management (CNS/ATM). That’s major duty.
The need for such a network is unquestionable. But progress towards its ultimate realization has been slower than any of its proponents anticipated. This, despite the fact that there appears little or no opposition to the concept exists throughout the aviation community.
Status Report
To review the current status of ATN system developments and tests, and to chart the way ahead, a high level conference on the implementation of the ATN, called ATN-2000, was held in London in September.
The conference was attended by more than 240 government and industry representatives from 56 world nations. They included national civil aviation officials, military staff, air traffic control (ATC) specialists, airline personnel, airframe manufacturers, avionics suppliers, and representatives of other concerned parties, such as the National Business Aviation Association (NBAA) and the National Air Traffic Controllers Association (NATCA).
The conference’s fundamental starting point was a simple one: digital data links will be the essential elements in tomorrow’s ATC environment. Today’s air-to-ground voice communications are now seen as limiting future growth, and one of the ATN’s main thrusts is to unload routine messages–such as frequency changes, altimeter settings, etc.–from the voice channels. Currently, these are estimated to occupy 80% of all VHF com channel availability time, and these channels are approaching overload.
Already, most European ATC authorities have further divided the 25-KHz spacing between adjacent VHF voice channels into three channels separated by 8.33 KHz. They have mandated the use of these new channels in their upper airspace.
However, some observers regard this as purely an interim step. They foresee the need to move all routine messages to a data link medium, to give controllers and pilots more time to devote to more critical functions. It has been estimated that controllers can spend up to 30% of their time passing routine voice messages. And, although no figures are quoted for flight crews, the immediacy of an ATC communication, however simple, demands a prompt spoken acknowledgment, even under high workload conditions. That is so the controller can move on to pass other messages to other aircraft in the sector.
By comparison, a digital message on a cockpit display screen requires no more than a single keystroke by the crew to acknowledge its receipt and acceptance. As well, the message remains on the cockpit screen as confirmation of ATC’s instruction. No longer, perhaps, will pilots have to remember the message or give the most often uttered radio expression among aviators: "Say again." These and other benefits of the ATN’s controller/pilot data link communications (CPDLC) have already been successfully demonstrated in European flight tests and in exacting simulations at U.S. Federal Aviation Administration’s (FAA’s) Atlantic City, N.J., test center.
But controllers don’t just speak to pilots. At less automated control centers–and these are by far the majority in the world today–controllers still exchange voice messages with other controllers and other centers. Here, too, congestion is slowly building up, gradually impacting the quality of the service. As a result, the need for worldwide, almost instantaneous, data messaging is becoming critical.
ATN and Free Flight
The ATN also is to impact the future national and international Free Flight environments, where aircraft are forecast to fly on individual "user preferred trajectories." In this instance, the aircraft’s separate, but sometimes potentially conflicting, paths will be delineated by periodic, automatic downlinking of their identifications, positions, altitudes and other data. At the same time, their safe separation will be monitored by traffic managers–tomorrow’s equivalents of today’s controllers.
To speakers at ATN-2000, therefore, the air and surface benefits of the new ATN concept were clear. And, it was pointed out, the transition to the ATN also was becoming urgent; several presentations at ATN-2000 echoed the troubling forecast that current air traffic levels will double by 2015. But at the same time, a number of conference speakers stressed that voice communications between pilots and controllers may never be entirely eliminated. European and U.S. studies have confirmed that data links are unsuitable for quickly changing situations, e.g., in the terminal area, and in emergency situations. For the foreseeable future, then, voice will still be with us.
However, the unequivocal message from the London conference was that data link is coming, and it is no longer a question of if, but when. This prompted a key question at the conference: Who will, or should, make the first major nationwide or fleet-wide ATN investment? Industry-wide, acceptance that the ATN will be essential is virtually universal. But also universal is acceptance that the level of investment required to achieve worldwide air and ground implementation will be in the billions of dollars.
Should the ATC service providers commit to install extensive ground ATN infrastructures to encourage user transition, rather than the reverse? That seems likely. The airlines and other operators appear reluctant to commit to re-equipage solely on the promise that a ground infrastructure will be ready at the same time.
Two Key Questions
The operators persistently voice two key questions, neither of which has been fully answered: How much will the transition to the ATN cost? And what payback can be expected from the investment? Unfortunately, the ATN-2000 conference shed no light on these issues. In a typical exchange, Tom Holford of United Airlines–who stated that his airline considered the installation of data link systems in its entire Airbus A319/320 fleet–referred the question of costs to Airbus Industrie, the equipment supplier. An Airbus representative, in turn, stated that the company was still working on the figures.
In terms of costs, however, the unofficial consensus was that individual retrofit installations of early production ATN-compliant data link systems (none of which are yet available) in current Airbus and Boeing all-digital airplanes could be in the $300,000 to $400,000 region. Fleet fittings would, of course, cost less, as should factory installations in airframes under construction. Much of the cost is reported to be in the interface with existing on-board digital systems, including computers dedicated to specific functions.
United’s Holford noted that the data link installation in the Boeing 747-400 required interfaces with no fewer than 20 separate on-board computers. And the cost of installing data link in older, non-digital airplanes, particularly airline widebodies like the DC-10, was described by one attendee as "out of sight." Another noted that if data link is the key to the future, that key must be "made of gold." Cost aside, some operators are anxious to adopt the ATN. An NBAA representative at the conference stated that several of his association’s members who fly "high end" corporate airplanes were following the data link situation closely and expected to be early ATN users. They are anxious "not to be left outside any future ATC environment," he added.
The Equipment Needed
The new ATN flight deck equipment has already been defined, and will include dual data communication display units (DCDUs), normally located at either side of the center console, plus conspicuous "ATC MSG" alerting lights on the coaming/glareshield in front of each pilot. And in the avionics bay, an entirely new device, the ATN router, would be installed, where it will determine the most efficient, and most cost-effective, communications downlink. It chooses between VHF, HF or satellite data links, in a manner totally transparent to the crew. In fact, outside terminal airspace, pilots will rarely select a communications mode or frequency in the ATN era.
The router is the heart of the ATN. Properly, however, that should be "hearts," and thousands of them, because routers will be found virtually everywhere in the worldwide ATN air and ground environments. At ATM centers, these extremely sophisticated units will instantly and unerringly direct masses of data to their destinations, whether several feet away to a nearby traffic manager’s position, or several hundred miles away to a distant control center.
Yet this level of automation has raised concerns about the loss of situational awareness, since pilots will no longer hear communications with other aircraft in the vicinity. As one speaker at ATN-2000 pointed out, they will also miss the subtle tonal variations of the controller’s voice in stressful circumstances. Observing rock-solid digits on a flight deck display could, it was suggested, lead to unwarranted complacency about what was happening in the surrounding airspace.
However, while ATN equipment costs are slowly coming into focus, answers to the airline’s second question about payback remain elusive. Flight operations staffs confirm that proving the investment payback case to their financial officials is extremely difficult. Because while no one disputes that, say, the CPDLC technique can, in principle, ease congestion and reduce delays, it is very hard for an individual operator to quantify such benefits in dollar terms.
Conversely, inaction is seen by some as a prudent move. As one attendee put it, "We should never have told the accountants that with CPDLC, the last airplanes to equip probably won’t need to," because, of course, VHF congestion is planned to be a thing of the past. In the end, the underlying key issue at the conference was that of how the ATN program should move ahead.
Would mandatory ATN equipage–similar to Europe’s mandatory requirements for 8.33-KHz-spaced VHF radios–move the process forward? A debate at the conference on this approach only served to exacerbate the divide between Europe, which tends to support mandatory equipage, and the United States, which rejects it outright. The Europeans feel that a critical mass of at least 30% of the aircraft flying, is required to see real ATC benefits from the ATN. They are prepared to impose a mandate, although they would much prefer to offer incentives to airlines, such as reduced user fees, to reach this goal.
The FAA’s Steve Zaidman made it clear that the United States will not support mandatory equipage. The U.S. position on the ATN mirrored its position on Free Flight technology: operators must be left to choose whether to take advantage of the system’s benefits.
Yet the United Airlines plan for fleet-fitting of its short-range domestic Airbus fleet underlined an alternative, financially justifiable way for the commercial carriers to slowly make the transition to ATN technology, both in U.S. and European airspace. This is to use ATN to dramatically improve the air carriers’ airline operational control (AOC) function. This vital communications network will have dedicated frequencies to carry each airline’s flight operations, engineering and similar essential messages. Currently, these functions are squeezed into the Airborne Communications Addressing and Recording System (ACARS) VHF system, which will, in a few years’ time, be unable to handle the load.
The slow transition envisaged by United reflects the reluctance of the airline industry at large to move directly to either of the two future, but technically dissimilar, digital data links that are now under development in the United States and Europe. These are called, respectively, VHF Digital Data Link (VDL) Mode 3 and VDL Mode 4. United plans to initially install the already proven VDL Mode 2, which also is being installed in six American Airlines B767-300s scheduled to join several major airlines currently involved in Eurocontrol ATN trials. While significantly less capable than the future modes 3 and 4, VDL Mode 2 offers the prospect of immediately expanding the throughput of ACARS by a factor of 10, according to industry officials at the conference.
Later, when modes 3 and 4 reach maturity, avionics upgrade modifications are expected to ease the future transition to the more advanced systems. And, although American Airlines is participating in the European ATC data link evaluation, a company spokesman at an International Air Transport Association (IATA) meeting in mid-2000 stated that the carrier’s main interest was in data link benefits for AOC, rather than ATC. Interestingly, at an airline/FAA meeting in September 2000, several major U.S. carriers reportedly expressed a preference for VDL Mode 2 combined with the 8.33 KHz channel spacing technique for domestic use, rather than waiting for FAA’s preferred VDL Mode 3 solution, called NEXCOM. Mode 3 presently is predicted for 2009.
Will It Start in Europe?
Again, there seems to be no question about the eventual need for the ATN. Whether the Europeans will achieve their desired critical mass of operators by 2005, as a Eurocontrol speaker stated, or whether a more realistic date will be 2010, as was also suggested, is an open question. But it seems certain that ATN’s first implementation will be in Europe, since this is where traffic delays are becoming critical and, consequently, where extensive test programs and the associated Link-2000 action plan have been advanced.
And as traffic conditions worsen, the Europeans could, in keeping with their earlier 8.33-KHz VHF channel spacing and their interference-rejecting ILS-receiver rules, conceivably introduce mandatory ATN equipage regulations. U.S. operators would be well advised to keep a watchful eye on developments across the Atlantic.